It is often desirable to be able to transmit audio and video signals over a twisted pair cable. In retail applications it is common for displays with integral speakers to be located at various locations within a shop. These displays are commonly driven from computers located in an equipment room that are connected to the displays via audio and video extension equipment that typically uses twisted pair cable.
In KVM (Keyboard, Video Monitor, Mouse) applications it is often useful to be able to transmit keyboard, video, mouse and audio signals over a single twisted pair cable.
The type of twisted pair cable that is commonly used for video extension applications is the same as that commonly used for networking applications. The cable consists of four individual twisted pairs each of which is designed to carry a differential signal. Three of the four twisted pairs can be used to carry red, green and blue analogue colour signals and the remaining pair can be used to carry digital data, which may include keyboard data, mouse data, RS232 data, digital audio data, USB data and control data associated with the operation of the extender.
It can be preferred to transmit audio signals digitally as analogue audio signals can be subject to noise which can be heard as hisses and crackles when no audio is being played. This is particularly so when analogue audio signals are multiplexed with other signals because it is hard to fully isolate the effect of the multiplexed signals on the analogue audio signals. Audio signals that are digitally sampled at 44.1 kHz with a 16 bit resolution give a good audio quality that is acceptable for many practical applications.
Keyboard, mouse, RS232 and extender control data typically requires a relatively slow signalling rate of a few hundred thousand bits per second. Signalling at this slow rate can be accomplished by connecting the I/O pins of a cheap, simple microprocessor (such as a PIC 16-series microprocessor as provided by MicroChip Inc.) to line drivers that are connected to the digital data twisted pair. However, digital audio data requires a higher signalling speed than can be readily achieved by this method. Therefore, when digital audio signals are to be transmitted an FPGA or programmable logic device can be used to provide the higher speed signalling and to multiplex the various signals together by interleaving the transmission of data for each device in sequence. For example a KVM extender that sends a video signal via three pairs and a sampled digital audio signal, multiplexed with other digital data over the fourth pair is the AdderLink X2-GOLD provided by Adder Technology Limited of Cambridge, United Kingdom.
Hence, it would be advantageous to be able to provide an extender device that enables an analogue video signal and a digital audio signal to be transmitted over a single twisted pair cable in a simpler way. It would also be beneficial if the extender device did not require the digital audio data to be multiplexed with other data on a data pair of the twisted pair cable. Further, an extender device that avoided the cost and complexity of data multiplexing circuitry would also be advantageous. Such an extender device would have utility in many applications. Such applications include video and audio extenders for the retail market where the manufacture cost of the extender is a significant factor and so a device that provides digital audio and analogue video extension at low cost is very advantageous.
A further significant application for such an extender device is in KVM switch systems that use twisted pair (CATx) cable. KVM switches enable one or more KVM consoles to access computers connected to the KVM switch. The KVM switch is typically connected directly to each computer's keyboard, mouse and video port using suitable cables. CATx style KVM switches are a variant of KVM switches. Instead of connecting directly to each computer, CATx KVM switches are connected via a CATx twisted pair cable to a small interface device, located near to the computer, that in turn connects to the computer's keyboard, video and mouse ports. The interface device contains a miniature circuit that converts the computer's native video, mouse and keyboard signals into signals that are suitable for transmission over the twisted pair cable and vice versa.
This is an advantageous approach for larger KVM switch systems because the cabling complexity in server rooms is simplified and interface devices can be built to connect to different types of computer, for example those with PS/2, USB or legacy Sun keyboard and mouse connectors. The common twisted pair cable interface simplifies the design of the KVM switch and enables smaller and neater KVM switches to be built with large numbers of ports. However, the commercial viability of the overall system is very sensitive to the cost of the interface devices as each computer requires an interface device.
It is desirable to support keyboard, mouse, video and audio signals on modem CATx KVM switches. It can therefore be seen that an interface device that provided this functionality whilst avoiding the need to implement costly circuitry to multiplex high speed audio data with keyboard and mouse data on the data pair of the cable would be advantageous.